The blood-ocular barrier system is formed by 2 main barriers: the blood-aqueous barrier and the blood-retinal barrier (BRB). The BRB is particularly tight and restrictive and is a physiologic barrier that regulates ion, protein, and water flux into and out of the retina. The BRB consists of inner and outer components, the inner BRB being formed of tight junctions between retinal capillary endothelial cells and the outer BRB of tight junctions between retinal pigment epithelial cells. The BRB is essential to maintaining the eye as a privileged site and is essential for normal visual function. Methods of clinical evaluation of the BRB are reviewed and new directions using optical coherence tomography are presented. Alterations of the BRB play a crucial role in the development of retinal diseases. The 2 most frequent and relevant retinal diseases, diabetic retinopathy and age-related macular degeneration (AMD), are directly associated with alterations of the BRB. Diabetic retinopathy is initiated by an alteration of the inner BRB and neovascular AMD is a result of an alteration of the outer BRB. Macular edema is a direct result of alterations of the BRB.

The blood-retinal barrier plays an important part in the processes of retinal pathophysiology. A new clinical method for the study of the blood-retinal barrier by vitreous fluorophotometry appears to satisfy the necessary requirements in that it is quantitative and shows good reproducibility and high sensitivity. The application of this method to a series of diabetic patients with apparently "normal" fundi revealed the presence of a significant breakdown of the blood-retinal barrier in the early stages of retinal involvement in diabetes. The extent of the breakdown can be measured allowing for comparative and evolutionary evaluations. The disturbance of the blood-retinal barrier, as evidence by vitreous fluorophotometry, appears before microaneurysms or capillary closure can be demonstrated by fluorescein angiography.

Optical coherence tomography (OCT) is a noninvasive imaging modality that allows detailed structural visualization of the retina. Optical coherence tomographic angiography (OCTA) is a functional extension of OCT that detects motion or blood flow contrast. Optical coherence tomographic angiography imaging has emerged as a noninvasive strategy to visualize the retinal and choroidal microvasculature without the use of an exogenous intravenous dye injection.^1,2 The technique is based on the principle of identifying the temporal evolution of the OCT signal caused by the motion of scattering particles such as erythrocytes within the vessels. This technique can be used to generate information on 3-dimensional blood flow for visualization of the retinal and choroidal vasculature. Fluorescein angiography and indocyanine green angiography are currently the criterion standards for visualization of vasculature. Fluorescein angiography provides functional information regarding leakage of dye to provide information on the permeability of vessels, as well as the ability to stain structures such as fibrous tissue or subretinal fluid, which is not available with OCTA. However, fluorescein angiography and indocyanine green angiography are invasive, have occasional adverse effects, and have limited ability to distinguish signals from different axial depths within the retina.

Purpose. Nitric oxide (NO) is involved in leukostasis and blood-retinal barrier (BRB) breakdown in the early stages of diabetic retinopathy (DR), but it is unclear which NO synthase (NOS) isoforms are primarily involved. In this study, the authors aimed to clarify the involvement of constitutive (eNOS, nNOS) and inducible NOS (iNOS) isoforms and the mechanisms underlying NO-mediated leukostasis and BRB breakdown. methods. Diabetes was induced with streptozotocin for 2 weeks. Mice were treated with a NOS inhibitor, N ^G-nitro-L-arginine methyl ester (L-NAME), which shows a preference for constitutive isoforms over iNOS. Vessel leakage was assessed with Evans blue. Leukostasis was quantified in flat-mounted retinas with confocal microscopy, in vivo with a scanning laser ophthalmoscope, and in vitro in a retinal endothelial cell line. ICAM-1, occludin, and ZO-1 levels were assessed by Western blot, flow cytometry, or immunohistochemistry. Nitrotyrosine content was assessed by immunohistochemistry. results. Diabetes increased leukostasis within retinal vessels and BRB permeability, which were reduced by L-NAME. Similar effects were observed in diabetic iNOS knockout mice. In diabetic mouse retinas, ICAM-1 protein levels increased, whereas the immunoreactivity of tight junction proteins, occludin and ZO-1 decreased, in correlation with increased protein levels of all NOS isoforms. Those effects were prevented by L-NAME and also in diabetic iNOS knockout mice. High glucose and nitrosative/oxidative stress also increased leukostasis caused by ICAM-1 upregulation. conclusions. These results indicate that the iNOS isoform plays a predominant role in leukostasis and BRB breakdown. The mechanism involves ICAM-1 upregulation and tight junction protein downregulation.